Electro-acoustic transducer



United States Patent Stephen L. Heidrich 29 Richmond Drive, Darien, Conn. 06820 629,623

Apr. 10, 1967 Dec. 22, 1970 lnventor Appl. No. Filed Patented ELECTRO-ACOUST 1C TRANSDUCER 13 Claims, 4 Drawing Figs.

US. Cl. 179/1155, 1 8 H31 Int. Cl. H041- 9/06 FieldofSeareh 179/101,

108,109,1E; 181/31, 31.1, (inquired); 179/] 15.5

v si lllllllllllg -i Ah [56] References Cited UNITED STATES PATENTS 2,549,963 4/1951 Boer 179/1 15.5 2,773,130 12/1956 Olson l79/1l5.5 2,761,912 9/1956 Touger 179/101 Primary Examiner-Ralph D. Blakeslee Attorney-Ernest G. Montague PA TENTEU 0&0221970 3,549,829

sum 1 OF 2 INVENTOR STEPHEN L. HEIDRICH BY ATTORNEY.

. 1 ELECI'RQ-ACOUSTIC TRANSDUCER The present invention relatesto sound translating devices, in general, and to electroacoustic high impedance transducers which consist of an Operable acoustic diaphragm with operat' ing means in a housing enclosing air or a fluid containing suitable acoustic properties wherein the enclosed fluid provides an acoustical load to the operations of the diaphragm which opposes the applied power variations more orless as determined by the acoustical properties of the enclosed fluid, in particular.

Loud speaker enclosures commonly used are basically of two types: the completely enclosed type known as the infinite bafile enclosure, and the vented type known as the reflex enclosure. The present invention relates to the infinite baffle or completely enclosed type. A characteristic of the infinite baffle is that the low frequency response capability is increased with an increase in size of the enclosed space. For a satisfactory response at 50 cycles this requires an enclosed space equivalent to approximately times the cubed nominal diameter of the acoustic diaphragm.- If the enclosed space is packed with fiber glass, as disclosed in US. Pat. No. 2,775,309 to EM. Villchur, an enclosed space equivalent to approximately 2% times the cubed nominal diameter of the acoustic diaphragm will produce an equally satisfactory response at 50 cycles, thus attaining a substantial reduction in size of the speaker enclosure. These devices are relatively low impedance acoustic devices;

The sound translating device disclosed in the Villchur patent may be regarded as utilizing an acoustic capacitance and an acoustical resistance couple or network, effective at low frequencies. in which the capacitance acoustically charged and discharged from applied'fiuid compressions and decompressions. generated from the back of the related acoustic diaphragm is retarded by the acoustical resistance of a fiber glass packing therein effecting ajtime lag that may be considered analogous to certain RC timing circuits used in certain electronic devices. The nature of the fiber glass resistance, however, is such that it is not effective as a means of further lowering the frequency response by means of increasing its density beyond a certain optimum for a given volume of enclosure.

The quest is for more compact and better speakers.

Accordingly, the present inventionprovides very compact loud speakers or other electroacoustic reproducing devices having a satisfactory wide frequency range of response with an enclosure of appreciably less than one times the nominal diameter of the acoustic diaphragm cubed. The present invention, due to high pressures and small volumes, is regarded as a high impedance acoustic device, or sound cell.

It is one object of the present invention to provide an electroacoustic high impedance transducer, wherein a satisfactory wide range of audio response from va suitable operable acoustic diaphragm mounted in a very compact housing or enclosure is 68 in which small air chambers'functioning as air capacitancesare interconnected through an extraordinary.

high acoustic resistance due to viscosity in an air duct conductor defined by'contiguously opposing walls of great area as compared with the cross sectional area of the duct and which in cooperation with the smaller of the air-capacitances or the acoustic regeneration chamber acts as an acoustic resistancecapacitancenetwork couple that reacts favorably to low frequency cycles of compression and decompression that are impressed via the first or acoustic. pressure generation chamber from the back of a related acoustic diaphragm.

The present invention is also directed to telephone receiver constructions, as well as to electroacoustic transducers in general, of the completely enclosed or infinite baffle type containing a resurgent high pressure acoustic system in which a high acoustic resistance is coupled with relatively small acousticcapacitances in response to lower frequencies of the acoustic spectrum of the general type as disclosed in my copending US. Pat. applications, Ser. No. 524,015, filed Feb. 1, 1966, entitled Electroacoustic Transducers, (now US.

Pat. No. 3,317,000 granted May 3, 1967), Ser. No. 586,240, filed Oct. 12, 1966, entitled Electroacoustic Transducers" (now US. Pat. No. 3,324,966 granted Jun. 13, 1967') and Ser. No. 619,658, filed Mar. '1, 1967, entitled Electroacoustic Transducer. I

In the aforementioned copending patent applications there is disclosed an enclosed acousticalsystem inwhich the related acoustic diaphragm is predominantly controlled by the acoustic characteristics of the enclosed air or fluid in the housing according to the properties of a system of acoustic elements consisting of fiuid compliance, resistance and mass contained in an arrangement of chambers and connecting duct or ducts; which improves the fidelity of the reproduced sound corresponding to the variations of electrical input.

As disclosed in the copending patent applications, the principal feature is in utilization of the resurgence of the resurgence of acoustic energy from a highly compressed fluid in a small secondary chamber controlled through a high fluid flow resistant duct back to the larger primary chamber or enclosure, in which the acoustic pressure variations are generated on the back of the related acoustic diaphragm, thereby causing the fluid resurgence to react effectively on the back of the acoustic diaphragm increasing the efficiencyof operation, that is, an acoustic resistance-compliance network designed to reproduce optimum acoustic response comparable to the electrical input.

In particular, the low frequency response capability of the infinite baffle is increased, for a'decrease in the acoustic capacitance heretofore contained in the enclosure, for very compact loudspeakers or receivers. This is achieved as indicated, with an operative acoustic diaphragm mounted in a small housing in which two very small air chambers functioning as air capacitances are interconnected by an extraordinarily-high acoustic resistance in the form of a high resistance air duct conductor of substantial ,area relative to its width between walls, which in cooperation with one of the two air capacitances acts as an acoustic resistance-capacitance network couple, that reacts favorably to low frequency cycles of compression and decompressiont-hat are impressed via the first chamber from the backof a related acoustic diaphragm.

The specific constructions disclosed in the first-mentioned copending patent application consist of an arrangement of contiguous walls forming an air conductor passage which is generally peripheral to the unit and axially disposed thereto. The specific constructions disclosed in the second-mentioned patent application consist of an arrangement of contiguous walls forming an annularly radialfluid medium passage disposed perpendicularly and radially to the unit; and wherein further, two annular chambers are formed about the magnetic structure of the unit permitting more compact loudspeaker systems. The third-mentioned copending patent application generally discloses an arrangement of wall means disposed in an enclosed space at the back of the diaphragm, which wall means may also include the fluidtight enclosure, or be separate therefrom, and which Wall means forms contiguous walls defining a narrow space therebetween, which narrow space communicates with the fluid medium in the enclosed space. Specifically, this latter patentv application discloses a telephone receiver or the like, comprising an acoustic resurgence network assembly defining two chambers connected by a high resistance duct formed by adjacent walls having a large surface area but narrow width between the walls, one of the chambers being immediately adjacent the rear of the diaphragm and the secondary or remote chamber communicating therewith via the narrow 'duct. The remote or regeneration chamber has a small volume compared to that of the first or pressure generation chamber adjacent the diaphragm providing a useful fluid resurgence to the operation of the diaphragm, and both chambers and the duct are in fluidtight isolation from the remainder of the unit.

However, particularly, the present invention is directed toward constructional and operational improvements in the annularly radial duct transducer devices providing more simplified, more economical and more precise alignment of the extremely narrow duct spacing.

Spacer elementshave been disclosed in the second-mentioned patent application disposed circularly between the annularly radial walls, defining the duct spacing, and are secured therebetween by bolt fasteners attached through the annular walls and the spacer elements. The entire spacer elements were positioned betweenthe secondary or rearmost chamber, and "the first chamber, i.e., entirely within the duct passageway, and further the contiguous walls defining the second-chamber themselves provided the airtight enclosure for the secondary chamber.

It is one object of the present invention to provide electroacoustic transducers of the annularly radial duct type having improved constructions and arrangements utilizing special spacer means and providing economies of manufacture, rapid and precise assembly, and improved acoustic response.

It is another object of the present invention to provide electroacoustic transducers of the annularly radial duct type in accordance with the above-mentioned objective wherein two contiguous radially oriented walls 'define therebetween the duct and a small secondary chamber, and wherein a spacer means is provided comprising at least'in part a full 360 circuniferential annulus or disc positioned between the walls at the remote end of the secondary chamber, and most remote from the first chamber or enclosure space which is rearward of the diaphragm, for not only determining precisely the duct spacing, but also for sealing the secondary chamber from fluid leakage and requiring to the fluid to responsively react back through the duct from the secondary chamber to the first chamber or enclosure space for optimum acoustic response on the :diaphragm. The spacer means may also consist of raised embossings on one or the other walls.

lt'is still another object of the present invention to provide electroacoustic transducers of the annularly radial duct type in accordance with the above-mentioned objective wherein the spacer means are sealed in place with pressure sensitive or thermal adhesive, or held by pressure means.

. It is yet another object of the present invention to provide electroacoustic transducers of the annularly radial duct type on accordance with the second-mentioned objective, wherein the spacer means includes a plurality of arcuately curved spokes extending from the outer circumference of the circumferential annulus or disc thereof into the duct passageway to insure uniform duct spacing.

With these and other objects in view which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings, in which: I

FIG. I is an axial section of an acoustic transducer embodying the feature of the present invention;

FIG. 2 is a section, along the lines 2-2, ofFIG. 1;

FIG. 3 is an enlarged axial section of an electroacoustic transducer in accordance with the present invention for a telephone receiver, or the like; and FIG. 4 is an axial section broken away in part of a telephone receiver showing the electroacoustic transducer of FIG. 3 therein.

,Referring'now to the drawings and in particular to FIGS. 1 and 2, the electroacoustic transducer of the present invention comprises a conventional dynamic loud speaker unit consisting ,of a conical acoustic diaphragm l suspended by a compliant peripheral flange 2 cemented to a supporting rim 3 of a basket 4 of a magnetic structure 5 actuated by a voice coil 6 which operates in a magnetic gap 7 in the magnetic structure 5. A cap 5a may be provided to seal and prevent fluid leakage in the clearance space between the voice coil 6 and the magnet poles. The dynamic loud speaker unit is integrated in a housing enclosure member 8 according to this acoustic device. A gasket 10a, preferably of rubber, is disposed about the front end of the housing enclosure member 8 and the basket 4 or the diaphragm 1.

member or platen 30' comprising .fa radial'annular wall 32 is I disposed about the magnetic structures and is perpendicular to the housing enclosure member.8,.'The annular radial wall 32 extends radially to and contiguously spaced from a similarly oriented annular wall or matrix 33 being asecond member 31 forming a thin air cell. The'latter member-"31' is disposed perpendicularly about the magnetic structure Sby a cylindrical flange portion 12, in a fluid tight forced fit and extends radially from the magnetic structure 5 nearly up to, but short of,

the front cylindrical portion 8a of the housing enclosure member 8a and isthen bent cylindrically rearwardly to abut a rear sealing gasket 10b, thereby defining a fluid tight, isolated,

rearmost chamber 8f. The magnetic structure 5 is preferably axially secured to the circular back wall 8d of the housing enclosure member 8 and spaced therefrom by a screw 8e or the like. The magnetic structure 5, so secured, thereby, holds the annular baffle member 30' in fixed and spaced circumferential relationship relative to the cylindrical wall 8a of the housing enclosure member 8. Y

The members 30' and 31' divide the housing enclosure member 8 further into a first and second chamber 30a and 30b, respectively, in which fiber glass packing 20 may be provided optionally to optimize the density for suitable damping. The contiguous annular radial walls 32 and 33 together define an annular duct 34 or thin air cell oriented perpendicularly and radially relative to the magnetic structure 5, communicating with the first and second chambers30a and 30b, respectively.

A spacing element 24 (FIG. 2) is circularly disposed abuttingly and sealingly between the baffle member 30 and the baffle member 31' and may be secured therebetween by pressure sensitive or thermal adhesive on both sides to insure the proper duct spacing and sealing of the second chamber 30b as will become apparent.

The spacing element 24 comprises anannular portion 251 having inner and outer circumferential edges 25a and 25b, respectively, disposed about and perpendicular to the mag netic structure 5, and adjacenttoand disposed radially inwardly of the second chamber30l7, respectively. A plurality of arcuate spokes 26 (four preferred) outwardly extend from the outer circumferential edge 25b of the annular portion 25 of the spacing element 24 substantially into the air conductor duct 34 terminating adjacent the outer peripheral edges of the contiguous walls 30 and 31, respectively, to further insure the integrity of the spacing of the duct 34 which is thereby maintained with precision as required. The curved spokes provide an increased abutment length for optimum spacing of the walls, of a length greater than that of the radial length of the annular duct and being narrow, do not interfere with the fluid passage through the duct.

A back loading acoustic resurgence network assembly in accordance with the'present invention is thereby formed constituting a pair of cooperating annular plates or contiguous members 30' and 31', which may be termed platen and matrix, respectively, which define the first chamber 30a immediately adjacent and behind the diaphragm, and the second chamber 30b, an annular fluid resurgence chamber, between the members surrounding the magnetic structure 5, both chambers communicating with each other via the radially extending annular duct 34 between the members. Both chambers or cavities 3tl'a and 3012 are isolated from fluid communication with the rearmost chamber 8f (which chamber may be eliminated by forming the member 31 as the exclusive rear wall of the housing and eliminating housing walls 3b, 8c and 8d therefor).

Specifically, the first baffle member or platen, defines with the housing enclosure member '8 the front chamber 30a immediately behind the diaphragm l and the second member or matrix 31 is annularly recessed, so as to define a rear annular chamber 30b with the rear of the first'baffle member 30, the first baffle member 30' having a perfectly planar radial annular wall 32 and the second member-"31' having the recessed radial annular wall 33, facing one another. The walls 32 and 33 are spaced apart so as to form theair duct 34 and the walls 32 and 33 have a substantial surfacearea compared to the spacing thercbetween or the width of the duct 34.

The acoustic resurgence. network assembly consisting of the members 30' and 31, which members-are sealed together inwardly of the second annular cavity 30b; by the spacing element 24, forms a fluid tight enclosure for the cavity 30b, isolating thecavityfluid therethrough.

This unitzperrnitsafluidmedium behind the diaphragm l to pass only between the cavities 30aand'30b as will hereinafter be described through the radially disposed annular duct 34.

The facing walls 32 and 33, respectively, of members30 and 31. respectively, are perfectly radially aligned by. the spacing element 24 and the member 31 is recessed at the inner radial" end of. the. duct 34 to form the annular chamber 30babout and spaced from the magnetic structure 5. it should also be noted that it is within the scope of the present invention that the chamber 30b may constitute the inner or remote end of the duct 34 without any cavity formation. a

The relative sizes of the chambers 30a" and 30b, respectively, are of importance to thefunctioning of the device although the invention is not restricted thereto. The total volume of the chamber 30b between the facing wall 32 and 33 is approximately 0.05 try-0.005 of the volume of the chamber 30a, being a fraction of the' volume of the chamber- 30a, which ratio applies not only. to telephone receivers but also to speakers, The acoustic resurgence network assembly of the invention provides a high compression chamber, namely chamber 30b, and a high resistance conduit 34 between the chambers. The baffle member 30' and. the radial annular wall 32 thereof extend radially, parallel and contiguous to the annular wall 33 of the member 31', nearly up to, but short of the housing enclosure member 8 to definea circumferential orifice at the circumferential edge of the annular wall 32 for fluid communication with the duct 34.

It is to be noted that the annular walls 32 and 33, and chamber connecting duct 34 are. oriented perpendicular and intermediate radially relative to the axis of the unit, sothat the first and second chambers 30a and30b, respectively, are sub.- stantially and annularly located with-respect to the magnetic structure 5, thereby optimizing space requirements of the system.

In operation the movements of the diaphragm 1 impelled by the energized voice coil 6, generate compressions and decompressions of air in the chamber 30a which being of a small volume causes plus and minus pressures to attain exceptionally high degrees above and below the atmospheric pressure and, thereby, provides an air stiffness and control over the movements of. the diaphragm 1, which predominates over the mechanical stiffness of the compliances in the diaphragm 1. The air stiffness thereby takes overmuch .of the inertia of the moving parts. 7 t

The air duct conductor 34 slowly releases the higher pressures in the chamber 30a, which are generated increasingly with decrease of frequency to the chamber 3017, where plus and minus pressures are regenerated again but with a time lag as compared with the pressures prevailing in'the first chamber 304 and reach a peak when the incoming air pressures through the air duct conductor 34 become equal to the'air pressures in the second chamber 30b. The regenerated pressures in the chamber 30b. which act as an acoustic buffer unit for low frequencies. become immediately resurgent with the reversal-of air flow adding the released power of resurgence through this air duct conductor. 34 to'the back of the acoustic diaphragm l at a frequency compatible to that in the acoustic resistance-capacitance network couple of the air duct conductor 34 with the air chamber 30b. 7

Because of the small housing desired. for this loud speaker, the acoustic capacitance of the chambers 30a and 30b is comparatively minute and the complementary acoustic resistance necessary to achieve a low frequency resurgence of 50 cycles or less is extraordinarily great. The high acoustic resistance is achieved in accordance with thegextremely close contiguous radial spacing, that is, in the order of a few thousandths of an inch, extending over a range of about one-tenth of a thousandth to about one hundred thousandths of an inch, depending upon the size of the loud speaker, between the annular walls 32 and 33 of the members 30 and 31, respectively, which define the air duct conductor 34, thus providingthe means for attaining an extraordinarily high acoustic resistance in the great area of the duct conductor 34. With the present radial spacing the perfectly planar bafile member 30' can be economically mass produced and the spacing element 24 further insures the precision alignment and narrow spacing of the duct 34. With this arrangementthere is applied a high pressure by virtue of the small capacitances, resistance controlled acoustic loading system in which the acoustic resistance is very great and is an effective retarding element, as well as of economical construction, to the flow of air between the two relatively small chambers. Such high acoustic resistance is obtained by the annular air duct 34 of the viscosity effect of extensive surface area as compared to the cross-sectional area of the duct. Accordingly, a high pressure resistance controlled regenerative acoustic loading system is achieved acting on the back of a related acoustic diaphragm, in which a high acoustic resistance is provided in the great viscosity residing in a thin radial air duct conductor formed intermediate the annular contiguous walls of substantial length as compared to the cross-sectional area of such air duct conductor. Referring now to the drawings and in particular to FIGS. 3 and 4, a telephone receiver is shown, which consists of a disc acoustic diaphragm 11 secured to an annular supporting rim 13 of a die-cast frame 14. The die-cast frame 14 fits into a conventional insulating material or plastic't elephone handle or shell 15 against a rear annular flange 16 thereof, and is secured in position by a conventional plastic receiver cap or ear cap 17, which is conventionally threadedly secured to the telephone handle 15. Two pole pieces 18, having rectangular portions 18a extending parallel to one another, terminate in pole tips 18b at one end thereof facing the diaphragm 11. The pole pieces 18 also include yoke portions 180, which yoke portions 18c are bolted to projecting lugs 14a of the die-cast frame 14, for securing the pole pieces 18 to the frame 14. The die-cast frame 14 has an annular supporting rim l9 rearwardly spaced from the diaphragm supporting riml3 and to which rim 19 is secured a baffle plate 31 having rectangular openings 35 therein, through which the pole tips 18b extend.

Voice coils 21 are wound about the rectangular portion 18a of the pole pieces and are electrically connected to contacts on a plate 21a, which plate 21a is also bolted to the projecting lugs 14a of the die-cast frame 14 with the yoke portions 180 of the pole pieces 18 sandwiched between the plate 214 and the projecting lugs 14a. Two bar magnets 22 (only one being shown) are welded at their ends in parallel relationship to laterally extending edges (not shown) of the yoke portions 18c.

extending beyond the projecting lugs 140.

All elements illustrated with the exception of the telephone handle 15 and the receiver cap'l7 constitute a receiver,

proper. A metal grid 23 covers the front of the receiver proper and is fastened to the die-cast frame 14 by thin ferrules (not 11. A resilient screen disc of impregnated silk (not shown) is The receiver cap 17 contains small holes 17a therein, which holes 17a constitute a front mesh designed with acoustic resistance, capacitance and mass to affect the acoustic response :of the diaphragm in the upper frequency ranges along with the acoustic compliance of the entire region between the receiver cap 17 and the diaphragm 11, which region constitutes an open ear cavity 9.

The electroacoustic transducer telephone receiver of the present invention includes a back loading acoustic resurgence network assembly comprising a pair of cooperating circular plates or members30 and 31, which define a first chamber 30a immediately adjacent and behind the diaphragm l1 and an annular fluid resurgence chamber 30b constituting a second or remote chamber between the plates surrounding the pole tips 18b, both chambers communicating with each other via a radially extending annular duct 34 between the plates. Both chambers or cavities 30a and 30b are located in front of the coils 21, in the receiver recess a and are isolated from fluid communication with the recess 15a. in this manner the recess 15a constituting an enlarged area, does not provide any fluid communication or acoustical response to the diaphragm 11 and is not used. v

. Specifically, the acoustic resurgence network assembly constitutes two cooperative, overlapping members, namely the first baffle member 30 defining with the die-cast frame 14 a front chamber 300 immediately behind the diaphragm 11 and a second member 31 defining a rear annular chamber 30b with the rear of the first baffle member 30, the first baffle member 30 having a radial annular wall 32 and the second member 31 having a radial annular wall 33, facing'one another. The walls 32 and 33 are spaced apart so as to form the air duct 34 and the walls 32 and 33 have a substantial surface area compared to the spacing therebetween or width of the duct 34.

A substantially disc-shaped spacing element 27 is disposed between members 30 and 31, inwardly and adjacent the innermost remote portion of the second cavity 30b, and about the pole tips 18b, and is sealed in place therebetween with pressure sensitive or thermal adhesive on both sides, so as to space the members 30 and 31 to precisely define the spacing for the duct 34 and to provide a fluid tight enclosure for the remote portion of cavity 30b. Also inwardly of the cavity 3%, the members 30, 31 and the spacing element 27 are formed with aligned holes 35 through which the pole tips 18b of the pole pieces 18 extend. The rear face 36 of the member 31 is sealed, in any suitable manner, to the rim 19 of the die-cast frame, thereby fluid tightly enclosing the opening in the die-cast frame and forming therein the first and second chambers 30a and 30b, respectively, communicating through the radial duct 34 between the members 30 and 31 and isolating the remainder of the telephone handle recess 15a from fluid communication therewith. The spacing element 27 includes a circumferential edge 27a aligned with the innermost remote end of the cavity 3012.

This unit permits a fluid medium behind the diaphragm 11 to pass only between the cavities 30a and 3012 as will hereinafter be described through the radially disposed annular duct34.

Only the facing wall of one of the members, herein member 31, is cut out and recessed at the inner radial end of the duct 3 4 to form the annular chamber 30b which is disposed about the pole tips 18b of the pole pieces 18, the other facing wall 32 being planar providing advantages heretofore mentioned. It should also be noted that it is within the scope of the present invention that the chamber 301; may constitute the inner or remote end of the duct 34 without any cavity formation, or that the other facing wall be recessed.

The relative sizes of the chambers 30a and 30b, respectively, as indicated, are of importance to the functioning of the device although the invention is not restricted thereto. The volume of the chamber 30b is approximately 0.05 to 0.005 of the volume of the chamber 30a, being a fraction of the volume of the chamber 30a. The acoustic resurgence network assembly of the invention provides a high compression chamber,

namely chamber 30b, and a high resistance conduit 34 between the chambers.

The baffle member 30 and the radial annular wall 32 thereof, extend radially parallel and contiguous up to the outer periphery of the annular wall 33 of the member 31, nearly up to, but short of the die-cast frame 14, and the outer periphery of the member 31 abuts an inwardly flanged shoulder 28 of the frame 14, to define a circumferential orifice 29 at the circumferential edge of the annular wall 32 for communication with the duct 34.

it is to be noted that th'e annular walls 32 and 33, and chamber connecting duct 34 are oriented perpendicular and intermediate radially relative to the axis of the unit, so that the first and second chambers 30a and 30b, respectively, are in front of the coils 21 and the second chamber30b is substantially and annularly located with respect to the pole tips 18b of the magnetic structure.

In operation the movements of the diaphragm 11 impelled by the energized coil 21, generate compressions and decompressions of air in the chamber 30a, which being of a small volume, cause plus and minus pressures, to attain exceptionally high degrees above and below theatrnos'pheric pressure and, thereby, provide an air stiffness and control over the movements of the diaphragm 11, which tends to predominate over the mechanical stiffness of the compliances in the diaphragm 11, the front chamber 9 and the receiver cap holes 170. The air stiffness thereby takes over much of the inertia of the moving parts. v

The air duct conductor 34 slowly releases the higher pressures in the chamber 30a, which are generated increasingly with decrease of frequency to the annular fluid resurgence chamber 30b, where plus and minus pressures are regenerated again but with a time lag as compared with the pressures prevailing in the first chamber 30a and reach a peak when the incoming air pressures through the air duct conductor 34 become equal to the air pressures in the second chamber 30b. The regenerated pressures in the chamber 30b, which act as an acoustic buffer unit for low frequencies, become immediately resurgent with the reversal'of air flow adding the released power of resurgence through this air duct conductor 34 to the back of the acoustic diaphragm 11 at a frequency compatible to that in the acoustic resistance-capacitance couple of the air duct conductor 34 with the air chamber 301;.

Because of the compact housing in telephone receiver shells, the acoustic capacitance of the chambers 30a and 30b is comparatively minute and the complementary acoustic resistance necessary to achieve a low frequency resurgence is extraordinarily great. The high acoustic resistance is achieved in accordance with the extremely close contiguous radial spacing, that is, in width in the order of a few thousandths of an inch, extending over a range of about one-half of a thousandth to about ten thousandths of an inch, preferably about 0.001 inch to 0.005 inch, depending upon the design of the telephone receiver unit, between the annular walls 32 and 33 of the members 30 and 31, respectively, which define the air duct conductor 34, thus providing the means for attaining an extraordinarily high acoustic resistance in the great area of surface viscosity provided as compared to the minute crosssectional area of the duct conductor 34-. With the radial spacing the baffle member 32 can be economically mass produced. The spacing element 27 is used to insure the precision alignment and narrow spacing of the duct 34 to seal the remote portion of cavity 30b. With this arrangement there is applied a high pressure by virtue of the small capacitances, resistance controlled acoustic loading system ,in which the acoustic resistance is very great and is an effective retarding element, as well as of economical construction, to the flow of air between the two relatively small chambers, in which the second chamber is a fraction of the volume of the first chamber. Such high acoustic resistance is obtained by the annular air duct 34 of the viscosity effect of extensive surface area as compared to the cross-sectional area of the duct. Accordingly, a high pressure resistance controlled regenerative acoustic loading system is achieved acting on the back of the acoustic diaphragm, in which a high acoustic resistance is provided in the great viscosity residing in a thin radial air duct conductor formed. intermediate .theannular contiguous walls of substantial.length as compared tothe crosssec'tional area of such air duct'conductor.

It is to be understood that other embodiments are possible and that the scope-of the present invention also may include any telephone receiver compact loud speaker or other acoustic device employing an operative acoustic diaphragm, the back of which acts on a system of two small interconnected chambers in which the generated and regenerated acoustical resurgencies from one to the other are controlled by a dominating high acoustic resistance in an air duct conductor defined by adjacent disposed wall surfaces of an extensive area, as compared with the cross-sectional area of the duct conductor. It is further understood that the term air is analogous with any fluid medium and the use of lighter or heavier than air fluid mediums in the resistance-capacitance system described, are within the scope of the present invention, which is defined by the objects and the claims. Also, the spacing'means or elements maybe embossed on one of the contiguous wall surfaces in accordance with the present invention, and the spacing element 24 of FIG. 1 may be adapted for use with the embodiment of FIGS. 3 and 4.

While I have disclosed several embodiments of the present invention, as applied to loud speakers, telephones or other receivers, it is to be understood that these embodiments are given by example only and not in a limiting sense.

lclaim:

l. A receiver electroacoustic transducer for generating acoustic pressure variations to afluid medium, comprising:

'a shell and ear receiver cap defining therein a recess;

a frame including a wall forming an opening and dispose in said recess;

means including a diaphragm and pole pieces for generating acoustic pressure variations to a fluid medium within said opening and disposed within said opening in said wall of said frame;

a first member and a second member disposed in said opening and rearwardly spaced from said diaphragm and cooperatively spaced dividing said opening into a first chamber immediately adjacent rearwardly of said diaphragm and a'second chamber, respectively;

a first radial wall of one of said members being disposed radially contiguous to a second radial wall of either one of said enclosures and defining in part a narrow radial air duct conductor between said first and second radial walls;

said first radial wall formed with a substantially annular recess defining said second chamber thereby and between said first and second members;

said second radial wall defining a perfectly planar radial sur face;

said first chamber communicating with said second chamber through said radial air duct conductor;

a circumferential spacer means disposed sealingly between said first member and said second member and circumferentially abutting in part said first and second radial walls adjacent said second chamber at a portion thereof most remote from said radial air duct conductor;

said radial air conductor being of substantial radial length relative to the axial distance between said first and second radial walls; and

said first and second members being isolated from communication with said recess of said'shell and ear receiver.

2. The electroacoustic transducer, as set forth in claim 1,

wherein:

said pole pieces include pole tips extending from the rear through said second and first members, respectively;

said second chamber being annular is disposed adjacent and about said pole tips,

said circumferential spacer means is disposed radially inwardly relative to said second chamber; and

0 wherein:

said second member is sealingly connected rearwardly of said diaphragm to said frame; and

said opening in said frame being rearwardly enclosed and isolated from communication with said recess in said'shell and ear receiver cap by said second member.

6. The electroacoustic transducer, as set forth in claim 1,

further comprising:

a circumferential spacer means disposed sealingly between said contiguous walls at a position most remote from said at least one open portion for circumferentially sealing said contiguous walls at said remote position and for spacing said contiguous walls, thereby defining said narrow radial air duct conductor space; and

adhesive means on said spacer means for sealing said spacer means between said contiguous walls.

7. The electroacoustic transducer, as set forth in claim 1,

further comprising:

a circumferential spacer means disposed sealingly between said contiguous walls at a position most remote from said at least one open portion for circumferentially sealing said contiguous walls at said remote position and for spacing said contiguous walls, thereby defining said narrow radial air duct conductor space; said enclosed space defines a first chamber immediately adjacent rearwardly of said acoustic'diaphragm and in fluid communication with said at least one open portion of said radial air duct conductor space; one of said contiguous walls defining a substantially annular recess facing the other contiguous wall, thereby defining a second annular chamber between said contiguous walls,

said other contiguous wall defining a perfectly planar radial surface facing said one of said contiguous walls; said first chamber communicating with said second chamber through said radial air duct conductor space;

said radial air duct conductor space being of substantial radial length relative to the axial distance between said contiguous walls; and

said circumferential spacer means disposed adjacent said second chamber at a portion thereof most remote from said radial air duct conductor space for circumferentially sealing said second chamber at said remote portion thereof.

8. The electroacoustic transducer, as set forth in claim 7,

wherein:

said second chamber is disposed radially inwardly relative said radial air duct conductor space;

said circumferential spacer means is disposed radially inwardly relative to said second chamber; and

said contiguous walls thereby being sealed together inwardly of said second chamber.

9. The electroacoustic transducer for generating acoustic pressure variations to a fluid medium, as set forth in claim 8, wherein:

said contiguous walls constitute at least in part annularly shaped walls;

said annularly shaped walls disposed at least in part about and perpendicularly to said magnetic structure, thereby locating said second chamber and said radial air duct conductor space annularly about said magnetic structure; and

said circumferential spacer means comprises at least in part a substantially annular portion disposed at least in part circumferentially about said magnetic structure.

said radial air duct conductor space.

12. The electroacoustic transducer, as set forth in claim 11, wherein said spokes are arcuate in shape.

l3. The electroacoustic transducer, as set forth in claim 12, wherein said spokes extend to the outer periphery of said contiguous walls. 

